implicits-usage-guide

Implicits Usage Guide

Implicits is a Swift library for implicit parameter passing through call stacks — eliminates "parameter drilling" (passing the same arguments through many function layers). Similar in spirit to implicit parameters in Scala or context receivers in Kotlin.

Core Pattern

func start() {
  let scope = ImplicitScope()
  defer { scope.end() }
  @Implicit var network = NetworkService()  // declare
  process(scope)
}

func process(_ scope: ImplicitScope) { fetch(scope) }

func fetch(_ scope: ImplicitScope) {
  @Implicit var network: NetworkService  // retrieve
  network.request()
}

Rules:

• ImplicitScope — last argument, no parameter label
• Always defer { scope.end() } after creating a scope
• Declaration: @Implicit var x = value (with initializer)
• Retrieval: @Implicit var x: Type (no initializer, explicit type)
• Missing implicit = compile-time error

Keys: Type-Based vs Named

Type-based (default) — the type itself is the key:

@Implicit var network: NetworkService

Named keys — use when type alone doesn't explain meaning:

extension ImplicitsKeys {
  static let guestMode = Key<Bool>()
}

@Implicit(\.guestMode) var guest = true   // declare
@Implicit(\.guestMode) var isGuest: Bool  // retrieve

Nested Scopes

Child scope inherits parent values, can override or add new (retrieval picks nearest):

func inner(_ outer: ImplicitScope) async {
  let scope = outer.nested()
  defer { scope.end() }

  @Implicit var extra = ExtraService()
  await extra.doWork(scope)
}

Async code is supported.

Closures

Closures don't inherit scope automatically. Use #withImplicits to capture implicits used in the body:

fetchData(completion: #withImplicits { result, scope in
  @Implicit var handler: ResultHandler
  handler.process(result)
})

Effects (async/throws) are inferred. @MainActor is preserved on the result; other global actors aren't — use named wrappers for those.

For full details on #withImplicits, named wrappers, capture lists, and #implicits, see docs/closures.md in the Implicits repo.

Factory Pattern

Store context for later use:

class Component {
  let implicits = #implicits

  init(_ scope: ImplicitScope) {}

  func createChild() -> Child {
    withScope(with: implicits) { scope in
      Child(scope)
    }
  }
}

Stored Properties in Types

@Implicit works as a stored property — the type needs a scope in its initializer:

struct View {
  @Implicit var theme: Theme

  init(_ scope: ImplicitScope) {}
}

withScope Alternative

withScope handles scope lifecycle when indentation isn't a problem and the body is small:

withScope { scope in ... }                    // root scope
withScope(with: implicits) { scope in ... }   // from captured implicits
withScope(nesting: outer) { scope in ... }    // nested scope

Build-Time Analysis Setup

Add the plugin to Package.swift:

.target(
  name: "MyApp",
  dependencies: [.product(name: "Implicits", package: "implicits")],
  plugins: [.plugin(name: "ImplicitsAnalysisPlugin", package: "implicits")]
)

Plugin graph rule: Every module on the path from your code to the Implicits library must have the plugin. Example: if App → FeatureA → FeatureB → Implicits and FeatureB uses implicits, then FeatureB and FeatureA both need the plugin (App too if it uses implicits).

Static Analysis Constraints

1. Type annotations may be needed for type-based keys — the analyzer infers when possible.
2. No dynamic dispatch — the analyzer can't track through protocol-typed values or closures stored in properties. Static calls only.